Propane Furnace - Diminishing Gas Supply After Ignition

We have a Lennox G26 propane furnace.

The gas valve gets solid >24vac and the ignitor ignites the gas. It stays burning for about 10 sec. then goes out.
A few seconds later, the gas valve solenoid closes.
This eliminates any issues with the controls (pressure sensor, flame sensor, temp sensors, etc.)
It HAS to be a problem with the supply from the propane tank or the valve inside the furnace. (also, I replaced the flame sensor already thinking that was the problem... it didn't fix it)
The initial blue flame used to last 10-15sec. yesterday. Today and only lasts 5sec. Each attempt after that only lasts 1sec.
Then after 3 attempts, the controls shut off and give a "burners won't ignite" error code.

Our tank is 500gal and about 4 months old. It is buried and has ~new regulators at the tank and at the house.

The gas company came out yesterday and installed a new regulator at the house. The furnace worked for ~20min.
Then when he left, the furnace shut off again and hasn't started since..

The gas company and a furnace company have both been out and they both say it's each others fault.
Gas says they are supplying the house fine, furnace says the gas supply chokes off after the gas line is being consumed.

The gas company is coming out again though, so hopefully they can check the pressure everywhere while the furnace is running to see where the problem is.

This is the coldest it has been this year, so that could have sparked the issue.

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If it was working and now isn't, get some more propane. The gauge may not be accurate.

Regulators do fail, but again, it was working. Changing the regulator should have fixed it in that case.

It could be either regulator.

Propane is stored as a liquid and "boils" off to supply the gas. When a tank gets low it may not be able to vaporize fast enough to give you enough gas. It shouldn't be a problem underground but again, the gauge may be faulty.

Hope it helps.

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The outside temperature today reached 40.
When I tried it at 3:00, IT WORKED!
It has been on every since then, so we aren't out of gas.
This really seems to be affected by the outside temperature.
I bet it shuts off later tonight when the outside temp drops.

What problems can be caused by temperature outside? Regulator problem?

It's a temperature pressure related issue, I'd have to think. Just like refrigerant, at higher temperature the pressure in the tank increases and thus allows the fuel to flow. When it's cold, not so much.

The furnace magically started working when temps raised to about 35F.
We have been waiting for our fuel delivery, but I bet we are low priority because we are at "20%".

Well guess what? The temps dropped into the 20s today and now our furnace doesn't work again!!!
I called in and told them we are at 10% (even though it really says 20%).
Even though I lied about the 10%, they still argued with me saying that 10% is enough to last for a while and they will get there today or tomorrow.

I had to yell at the lady on the phone, telling her WE ARE OUT OF GAS!!! even though it says 10% (lie), it is not vaporizing enough to supply our furnace!

It's 5:30 on Friday. I don't like my chances of getting a delivery this weekend...